Plague is an infectious disease that affects animals and humans. It is caused by the bacterium
Yersinia pestis. This bacterium is found in rodents and their fleas and occurs in many areas of
the world, including the United States. Y. pestis is easily destroyed by sunlight and
drying. Even so, when released into air, the bacterium will survive for up to one hour,
although this could vary depending on conditions. Pneumonic plague is one of several forms of
plague. Depending on circumstances, these forms may occur separately or in combination:

Pneumonic plague occurs when Y. pestis infects the lungs. This type of plague can
spread from person to person through the air. Transmission can take place if someone breathes in
aerosolized bacteria, which could happen in a bioterrorist attack. Pneumonic plague is also
spread by breathing in Y. pestis suspended in respiratory droplets from a person
(or animal) with pneumonic plague. Becoming infected in this way usually requires direct and
close contact with the ill person or animal. Pneumonic plague may also occur if a person with
bubonic or septicemic plague is untreated and the bacteria spread to the lungs.

Bubonic plague is the most common form of plague. This occurs when an infected flea bites a
person or when materials contaminated with Y. pestis enter through a break in a person's skin.
Patients develop swollen, tender lymph glands (called buboes) and fever, headache, chills, and
weakness. Bubonic plague does not spread from person to person.

Septicemic plague occurs when plague bacteria multiply in the blood. It can be a complication of
pneumonic or bubonic plague or it can occur by itself. When it occurs alone, it is caused in the
same ways as bubonic plague; however, buboes do not develop. Patients have fever, chills,
prostration, abdominal pain, shock, and bleeding into skin and other organs. Septicemic plague
does not spread from person to person.